This invention relates to a method of forming a coating of glass-like carbon on titanium metal.
Titanium metal is heat-resistant and high in specific strength. In the air, an oxide film is formed on its surface, making titanium metal corrosion-resistant. Pure titanium can be alloyed with any metal, especially copper, tin, iron, aluminum, vanadium, chromium, cobalt, molybdenum and nickel. By alloying, it is possible to improve its workability, mechanical strength and other properties.
Titanium metals, i.e. pure titanium and titanium alloys are active metals, so that their surfaces are oxidized soon in the air, forming a film of titanium oxide thereon. Protected by such titanium oxide films, the titanium metals show high corrosion resistance even under corrosive environments. Thus, titanium oxide films formed on titanium metals are called passivated films.
The present inventors developed a material useful as sliding members and tightening members such as bolts and nuts obtained by cleaning the surface of a titanium metal to remove an oxide film, followed by carburizing to reduce friction and wear coefficients. A method for manufacturing such a material is disclosed in unexamined Japanese patent publication 7-90542.
Glass-like carbon is a nondirectional aggregation of hexagonal structure with a side of several nanometers, which are basic units of a graphite material. It is a kind of carbon having a nearly amorphous structure with its graphite units never growing even when heated to around 3000.degree. C. (hard-to-graphitize carbon). Such highly amorphous glass-like carbon has various industrially desirable properties including high corrosion resistance, wear resistance, lubricity (low friction coefficient), releasability, and gas impermeability.
In a conventional method for forming a glass-like carbon article, a thermosetting resin such as phenol is molded into a desired shape, polymerized, thermally set, machined, and heated very slowly while strictly controlling temperature for carbonization.
It would be possible to form a glass-like carbon film on the surface of a metal using this conventional method, i.e. by forming a thermosetting resin coating on the metal surface and heating it for a long time while controlling the temperature.
But with this method, it is difficult to form a homogeneous glass-like carbon film on the surface of a titanium metal due to the formation of a layer in which substrate metal and glass-like carbon are mixed together or the formation of a carbide of substrate metal. It is also difficult to form a sufficiently thin glass-like carbon film on the metal surface with high efficiency.
An object of this invention is to form a highly amorphous and homogeneous glass-like carbon film on the surface of a titanium metal with high efficiency and desired thinness.
Another object is to provide a method of forming a coating of glass-like carbon on a titanium metal article with high efficiency to improve the corrosion resistance, wear resistance (friction coefficient), releasability, gas impermeability and bio-affinity of titanium metal, thereby providing a surface treatment method that can increase the industrial value of titanium metals.